2. Catalysts (See also references II/26 to II/30 at end of this section).
(b) Reduction.
At the present time, the following conditions are used for the reduction:
Pure hydrogen (Sulphur - 0.10 gm/100 m3) enters the system through a Pattenhausen grude tower to protect against break through of sulphur. The gas next passes a methanizer to remove traces of CO (the methanizer could however be omitted). The fresh H2 is then added to the recycle, where it represents 3-5% of the total stream. The total hydrogen passes through silicagel drums, is dried and the dry hydrogen is blown through the preheater and the reduction vessel. The exit gas is cooled with water and NH3 before being returned to the silicagel drier.
The reduction vessel (1400 mm. diameter) is filled with 500 liter fuzed Fe catalyst (0.5-1.0 mm.). The catalyst is retained on a bronze screen. Care is to be taken that no catalyst particles fall through the screen before reduction.
After thorough purging of the H2 - Circuit, the heater is fired and the temperature raised to 450° C within five hours. The dried H2 is circulated at 200 V/H/V.
The water content of this H2 after drying is 0.5-2 gm/m3. At the beginning of the reduction the H2O content after the catalyst rises to a dew point of plus 4.0° C. Part of the water is condensed in the NH3 cooler and remainder is removed in the driers. A certain amount of NH3 formed during the reduction is absorbed in the water.
The reduction is completed after 50 hours. The system is cooled and CO2 on the catalyst. This CO2 used for blanketing must be dried over silicagel and must be free of sulphur.
In handling utmost care must be taken to prevent all oxygen from coming in contact with the catalyst. Even the slightest poisoning of the reduced iron with air or oxygen requires a higher initial synthesis temperature (less alcohols) and shortens the catalyst life. The apparent density of the finished catalyst varies from 1.8 to 2.2 according to particle size.
The life of the catalyst is around 9 months. For the design of a plant, which was to make catalyst for a 10,000 ton/year synolplant, an average life of only 4 months had been assumed. It is note worthy that additions of several tenths of 1% of arsenic to the catalyst results in a substantial increase of esters (25% in the 200°-300° C fraction). In the synthesis the catalyst immediately forms Fe2 C of hexagonal crystal structure and above temperature of 290° C this carbide is converted to an inactive form.
Particle size: It is of interest to note that the change in catalyst particle size had considerable effect on the results. Towards the end, I.G. used sizes between 0.5 and 1.0 mm. This led to increased output from the unit catalyst volume. It was concluded that the inside of the particles did not participate in catalyzing the reaction.
The drop in catalyst size from 1-2 mm. to 0.5-1 mm. made it possible to lower the temperature form 198° to 191° C. This in turn increased the ratio of H2O:CO2 in the reaction products, since the shift decreases at lower temperature.
Synthesis in “fluid bed” reactors and fluid bed reduction tests had been scheduled but were never started.